Transmit apodization control for microbeamformers

ABSTRACT

Apodization control techniques for a microbeamformer including a plurality of microbeamformer channels each including a transducer, a microbeamformer transmitter for driving the transducer, a microbeamformer receiver for receiving signals from the transducer and usually a delay element for delaying the received transducer signals. To improve the generation of waveforms by the transducers, the voltage provided to the microbeamformer transmitters is adjusted and/or the current provided by the microbeamformer transmitters is adjusted. The microbeamformer channels can also be grouped together into patches and/or clusters with the patches and clusters being provided with a common voltage source or current.

The present invention relates generally to ultrasonic transducers formedical imaging and more particularly to a system for apodizationcontrol in an ultrasonic transducer, i.e., to control the shape of atransmit beam of an ultrasonic transducer in particular in the contextof sub-array beamforming.

Sub-array beamforming or microbeamforming involves the use of transmitand/or receive arrays of ultrasonic transducers grouped into sub-arrays.U.S. Pat. No. 5,997,479, incorporated by reference herein, describes oneapplication of microbeamforming in which a plurality of transducerelements are grouped into several transmit sub-arrays, and a receivearray includes a plurality of transducer elements grouped into severalreceive sub-arrays. FIG. 2 of the '479 patent also shows severalintra-group transmit processors, connected to the transmit sub-arrays,and which generate a transmit acoustic beam directed into a region ofinterest, and several intra-group receive processors connected to thereceive sub-arrays. Each intra-group receive processor is arranged toreceive, from the transducer elements of the connected sub-array,transducer signals in response to echoes from the transmit acousticbeam. Each intra-group receive processor includes delay and summingelements which delay and sum the received transducer signals. A receivebeamformer includes several processing channels connected to theintra-group receive processors, and each processing channel includes abeamformer delay which synthesizes receive beams from the echos bydelaying signals received from the intra-group receive processor, and abeamformer summer which receives and sum signals from the processingchannels.

Further, U.S. Pat. No. 6,013,032, incorporated by reference herein,describes another microbeamformer in which each sub-array of thetransducer array is connected to a sub-array beamformer with eachsub-array beamformer including a sub-array processor and a phase shiftnetwork connected to the output of the sub-array processor (see FIG. 2and the description thereof). A primary beamformer includes a summingunit which sums the outputs of beamformer channels to which the outputof the sub-array beamformers is provided, and thereby provides abeamformer signal that represents the received ultrasound energy along adesired scan line.

The term microbeamformer, as generally used hereafter, describes asub-array beamformer that is integrated within the handle of thetransducer in order to facilitate connection to a very large number ofpiezo-electric sensor elements arranged in a 2D array. Such aconfiguration allows for real-time volumetric imaging, when used incombination with a mainframe beamformer and back-end display subsystem.Instead of integrating the electronics of the sub-array beamformerwithin a handle of a transducer, they may be arranged in the mainframe.The term microbeamforming could also be applied to 1D arrays.

In microbeamforming, control of the shape of the transmit beam is animportant aspect for successful implementations of multi-line imagingtransducers, in particular, for real-time volume acquisition wherehigh-order multi-line imaging is required to achieve sufficient volumeacquisition rates. Control of the shape of the transmit beam is possiblebecause in current ultrasonic transducers, each element in thetransducer array is typically connected to control electronics so thateach element is individually controllable.

Also, in microbeamforming and other beamforming applications including atransducer array, only a portion of the total number of elements in thetransducer array may be operable at any time. This is referred to ascontrolling the aperture of the transducer array. The aperture of thetransducer array refers to the configuration of the transducer elementsthat are active at any moment. The electronic control of each element inthe transducer allows the transmit and receive signals to be shaped anddelayed to provide an appropriate signal for the type of imaging beingperformed.

Referring to FIG. 8, microbeamformers are often constructed with aplurality of microbeamformer patches 100 with each microbeamformer patch(or sub-array) 100 including at least one and most often a plurality ofmicrobeamformer channels 102. Each microbeamformer channel 102 isconnected to a transducer 106 and includes a microbeamformer transmitter104 for driving the transducer 106 and a microbeamformer receiver 108for receiving signals from the transducer. Preferably, a delay 110 isalso provided to delay the received transducer signals and a controlcircuit 122 is provided to stimulate the transmitter 104. Additionaldetails about the manner in which microbeamformer channels operate canbe found in the patents discussed above. Thus, the microbeamformer hasindividual transmitters for each microbeamformer channel 102 to providebeam steering and focusing control. While additional transmitters aretypically provided on the mainframe 112, these mainframe transmittersare not used to drive the transducers 106. Rather, coaxial cables 114are connected between receivers 116 on the mainframe 112 and themicrobeamformer patches 102 so that the coaxial cables 114 are only usedfor the receive path and not for the transmit path. A single powersupply 118 is coupled to all of the microbeamformer patches 100 via acoaxial cable 120.

In a microbeamformer such as shown in FIG. 8, it is known that bycontrolling the timing and transmit energy supplied to some or all ofthe microbeamformer channels (commonly referred to as “transmitbeamforming”), the ultrasonic interrogation pulse sent into an objectbeing examined can be shaped to provide, for example, high resolution atvarious depths. Similarly, by electronically altering the receiveweights and delays (referred to as “receive beamforming”), the receivedenergy can be used to form high quality images at various depths.

One manner for controlling the transducer elements is known asapodization. Apodization of an ultrasonic transducer aperture is agradual reduction of the transmit amplitude and/or receive gain from thecenter of the aperture to the edges of the aperture with a resultantdecrease in beam side lobe levels.

In practice, different apodization methods are applied. For example, itis known to use square wave pulsers with power supply voltages that varyacross the active aperture and it is also known to apply a per-channelapodization using wave-shaping transmitters. This capability is obtainedthrough additional complexity in either the power management componentsor the individual transmitters.

When designing microbeamformers for real-time 3D, space is at a premiumbecause the microbeamformer integrated circuits (ICs) must fit in thehandle of the transducer. In addition, power dissipation must be limitedbecause of the difficulty in providing cooling for the microbeamformerelectronics. As such, the transmitter in the microbeamformer should haveas simple and basic a construction as possible and complex modificationof the transmitter to provide apodization should be avoided.

The microbeamformer ICs in one prior art system use unipolar pulsersthat provide two levels of apodization on a per-element basis—on or off.There are drawbacks to this system most notably, the apodization islimited and often does not provide for adequate beam sidelobe control.It would thus be advantageous to provide new apodization controltechniques for transmission from microbeamformers which would allow foradequate beam sidelobe control without significantly complicating thecircuitry that must reside within the transducer handle.

To control the acoustic signal generated by the transducers, some priorart ultrasound imaging systems drive the array elements in thetransducer with a simple square wave (boxcar) type voltage excitationsignal of varying duration and duty cycle. It is known in the art how tocreate these voltage excitation signals given a fixed or variablemainframe power supply. Often, the voltage or pulse width is changed totry to alter the amplitude of the acoustic signal. Changing the drivevoltage changes the total power that can be supplied to drive thetransducer whereas changing the pulse width of the driving voltagealters the way the transducer resonates and different acoustic signalamplitudes are possible. For the purposes of apodization across anarray, having different drive voltages on every transducer works well.However, for those drivers commanded to output low voltages, the drivercircuits themselves dissipate a lot of energy since the output voltageand the system high voltage bus may be very different. Formicrobeamformers, this inefficiency cannot be tolerated (due to theassociated probe heating) so it would be advantageous to provide anefficient driving technique that allows for different output voltagepulses.

To generate a square wave voltage pulse to the transducer, a transmitterneeds to source or sink significant amounts of current in order tocharge up the capacitance associated with the transducer. Unfortunately,the current through pull-up and pull-down MOSFET devices is directlyproportional to their width, so a very large (wide) device is needed tosource or sink large currents. Since space is at a premium inmicrobeamformers, it would be advantageous to develop a pulsingtechnique that does not require large driver currents so smaller devicesmay be used.

It is known in the art of transducer design that the current supplied toa transducer is proportional to the velocity of the face of thattransducer and hence of the pressure (acoustic amplitude) developed inthe medium being transmitted into. In order to change the apodizationacross the array, it may be useful to exploit this sensitivity of thetransducer drive current while maintaining the relatively small size ofthe microbeamformer.

It is an object of the present invention to provide a new system forapodization control of an ultrasonic transducer array driven by amicrobeamformer.

It is another object of the present invention to provide a new systemfor apodization control of an ultrasonic transducer array driven by amicrobeamformer using pull-up/pull-down devices in conjunction withmultiple pulser power supply voltages.

It is yet another object of the present invention to provide apodizationcontrol using multiple switchable current sources to drive theultrasonic transducer.

It is still another object of the present invention to provideapodization control using one or more switchable current sources thatdrive the ultrasonic transducer for varying amounts of time.

In order to achieve these objects and others, a system for apodizationcontrol of a microbeamformer in accordance with the invention includes aplurality of microbeamformer channels grouped or allocated into aplurality of microbeamformer patches, each microbeamformer channelincluding a connection to a transducer, a microbeamformer transmitcontrol and driver circuit for exciting the transducer, amicrobeamformer receiver for receiving the transducer signals andusually a delay for delaying the received transducer signals. Themicrobeamformer transmitters in each patch are connected to a commonpower supply node but have individual timing control circuits. Amainframe beamformer has a plurality of mainframe channels eachincluding a mainframe receiver and a mainframe transmitter fortransmitting a pulsed voltage. Each microbeamformer patch is connectedto a respective mainframe channel, for example, by a cable connected tothe common node of the patch, such that the mainframe receiver inputssignals from the patch of microbeamformer receiver datapaths.

There several different ways to achieve the objectives of the presentinvention and they involve either providing unique power supply voltageconnections to the microbeamformer patches or providing unique driverelectronics to control the current driven to/from each associatedtransducer. The first technique uses mainframe transmit drivers tosupply each patch with a different high voltage power supply that isthen used to drive the individual transducers within that patch todifferent voltages. The second technique allocates a number of variablehigh voltage supplies in the mainframe (separate and in addition to themainframe transmit drivers) that are connected to one or more of themicrobeamformer patches. A third technique uses a single fixed highvoltage power supply for all the microbeamformer patches but providesdifferent current drive outputs per patch or per transducer to controlthe amplitude of the transmitted acoustic waveform. Finally, a fourthtechnique uses a single fixed high voltage power supply for all themicrobeamformer patches and a single current drive output, but thelength of time the current is asserted to the transducers is variable todeposit varying amounts of electrical power to the transducers and thusexcite different acoustic amplitudes. These techniques can be used andimplemented individually or in various combinations to achieve aplethora of different microbeamformer transmitter configurations withvarious drive capabilities. Each technique accomplishes the object ofthe invention to provide apodization control of the ultrasoundtransducer array and these are summarized below.

The first technique to apply apodization to microbeamformed arrays usesthe mainframe transmit drivers in the mainframe beamformer to supplyeach patch with a different high voltage power supply that is then usedto drive the individual transducers. Timing control circuits in eachmicrobeamformer channel determine when the associated microbeamformertransmitter within the patch excites the transducer. The mainframetransmitter channel voltage, and therefore the patch supply voltage, canbe varied arbitrarily in this configuration since the mainframetransmitters are designed to provide a wide range of output voltages.The transmitter in each mainframe channel may be arranged to transmit aunipolar pulse for the duration of each transmit burst by themicrobeamformer transmitters such that the pulses driven by themicrobeamformer transmitters have an amplitude equal to the unipolarpulse from the mainframe channel. In this manner, the mainframetransmitters become a variable power supply for the patch oftransmitters in the microbeamformer.

A complicating detail of this first technique involves separation of thetransmit events, which are inherently high voltage, and the receiveevents which are inherently low-voltage. Specifically, to preventtransmitter noise from contaminating the receive datapath, diodes areprovided between the microbeamformer transmitters and the cable andbetween the cable and the mainframe transmitters. Also, to protect thereceivers when the transmitters are operative, protection devices suchas switches can be provided both in the output path from themicrobeamformer receivers and in the input path of the mainframereceiver.

By providing an independent power supply voltage to each patch via amainframe transmitter, each patch can transmit different amplitudewaveforms and thereby provide for enhanced shaping of the transmit beam.

In another embodiment of the invention, the patches are grouped into aplurality of clusters with each cluster preferably including a pluralityof patches. Microbeamformer transmitters are then powered from themainframe by a plurality of individually adjustable power supplies (ormainframe transmitters) and each cluster is connected to a respectivepower supply, such as by a cable. The power supply voltage applied toeach cluster is independently adjustable or settable, for example, to adifferent voltage for each pulse repetition interval.

Although the transmitters in each microbeamformer patch are not poweredby a dedicated power supply as in the embodiment above, clusters ofmicrobeamformer patches are powered by a common power supply and byappropriate assignment of the microbeamformer patches to clusters, theshape of the transmit beam can be effectively and advantageouslycontrolled. Eliminating the diodes and switches is the primary benefitof this embodiment without dramatically increasing the number of cablesneeded to power the various patch transmitters.

It is a straightforward extension of this embodiment to include highvoltage switches (or other switching means) within the microbeamformertransmitters that can selectively choose from a small number ofmainframe-supplied high voltage power rails. That is, rather than havingthe patches or microbeamformer channels clustered together to share acommon high voltage supply each microbeamformer channel can include oneor more high voltage switches to select from a small number of powerrails. The selection of which power rail to use and, therefore, theapplied drive voltage can be made on a channel by channel basis and canbe different for each pulse repetition interval.

In a third embodiment of the invention, the microbeamformer transmitdrivers can be specifically designed to drive a particular transducerwith a variable current source rather than as a voltage source. Unlikemainframe transmitters that must be able to drive many different typesof transducers, the microbeamformer transmitters can be designed tooptimally and efficiently drive a specific transducer. As such, it istherefore possible to use a plurality of switchable current sources andsinks in each microbeamformer transmitter to drive the transducer, wherehigher currents excite larger acoustic amplitudes and smaller currentsexcite smaller acoustic amplitudes from the transducer. Theseapodization techniques can be accomplished using a single high voltagepower supply common to all microbeamformer channels. Moreover, thecontrol of the apodization by selecting specific source and sinkcurrents can easily be accomplished at each microbeamformer channelrather than being grouped into patches or clusters of patches.

This embodiment of the invention includes a plurality of switchablecurrent sources, each including a series switch and pull-up device aswell as a plurality of switchable current sinks, each including a seriesswitch and pull-down device. Such a configuration of switchable currentsources and sinks is known to those skilled in the art as a current modedigital to analog converter (DAC). The aforementioned pull-up devicescan be PMOSFET (also known as PMOS) devices that are biased to provideconstant currents from the high voltage supply to the transducer. ThePMOSFET devices can also be biased off, so as to act as a switch, or aseparate PMOSFET device may be allocated in series to accomplish theswitch function. Similarly the pull-down devices can be NMOSFET (alsoknown as NMOS) devices that are biased to provide constant currents fromthe transducer down to ground (or a negative supply). The NMOSFETdevices can also be biased off, so as to act as a switch, or a separateNMOSFET device may be allocated in series to accomplish the switchfunction.

It is known to those skilled in the art that the current carryingcapacity of PMOS and NMOS devices is proportional to their width and isalso a function of the applied gate bias. Therefore, in this embodiment,the microbeamformer transmitter would preferably include a small numberof PMOS and NMOS devices that can be selectively enabled (by their gatebias) to supply various drive currents to and from each transducer. Formaximum efficiency, pull-up and pull-down devices may not be conductingcurrent at the same time, however, different, less-efficient modes ofoperation are possible.

During normal operation of a preferred embodiment of the invention, aselected subset of PMOS devices will be enabled to drive the transducer.The number of devices enabled and therefore the available drive currentcan be controlled on each microbeamformer channel individually. The PMOSdevices will preferably be enabled for a duration of approximately onequarter of a wavelength of the acoustic signal that is to be excited.Subsequently, a selected subset of the NMOS devices will be enabled todischarge the transducer for a duration equal approximately to onequarter of a wavelength of the acoustic signal being excited. Note,however, that if the selected pull-up and pull-down currents are notequal, the durations of the pull-up and pull-down events may not beequal. The voltage developed on the transducer is a function of theapplied drive current and the capacitance of the transducer, but it willin general be a ramp up to some voltage followed by a ramp back down.The slope of these ramps is defined by the driver current and transducercapacitance.

It is a key component of this embodiment of the invention that the drivecurrents during pull-up and pull-down events be programmable but thatthe duration of these events be fixed (for a particular desired acousticfrequency). That is, to accomplish different apodization levels acrossthe array, each of the microbeamformer channels may drive differentcurrents for a specified pull-up and pull-down duration that is commonamong all channels. The time at which the channels drive the transducersdepends on the desired delay and the selected current driven to thetransducers depends on the desired apodization.

It is a straightforward extension of the embodiment above to provide yetanother embodiment of the invention where one pull-up and one pull-downdevice are allocated on every microbeamformer channel to drive thetransducer for durations of time that vary according to the desiredapodization across the array. This pulse-width-modulation approachvaries the amount of time the pull-up and pull-down devices are enabledand thus controls the total power delivered to the transducer. Theresultant acoustic waveform amplitude will be roughly proportional tothe width of the applied current pulses.

The key differences between this embodiment and the previousmulti-current embodiment is in the way in which the microbeamformertransmitters are controlled. In the multi-current embodiment, theapodization function specifies the current setting but the pulse-up andpulse-down sequence is the same among all channels (though the starttime of this sequence differs from channel to channel). In thisembodiment, the apodization function specifies the pulse widths of theup and down events but the pull-up and pull-down currents are the sameamong all channels (and again the start time of the sequence differsfrom channel to channel). Clearly there are tradeoffs in timing controlcomplexity between these embodiments and this must be traded off againsthigh voltage current source complexity (size) differences.

It should be evident to those skilled in the art that each of theseembodiments constitutes novel power supply connection paradigms, highvoltage driver circuit designs, and driver timing control techniquesthat could be combined in various different configurations. Dependingupon the application and implementation constraints, one of thesevarious embodiments or combinations thereof may be optimal or desired.This invention should be understood to cover the individual embodimentsdescribed as well as arbitrary combinations thereof

The invention, together with further objects and advantages hereof maybest be understood by reference to the following description taken inconjunction with the accompanying drawings, wherein like referencenumerals identify like elements and wherein:

FIG. 1 is a schematic of a first embodiment of a microbeamformer portionof an ultrasonic transducer using apodization control in accordance withthe invention.

FIG. 2 is a schematic of a second embodiment of a microbeamformerportion of an ultrasonic transducer using apodization control inaccordance with the invention.

FIG. 3 is a diagram showing a possible grouping of microbeamformerpatches into clusters in accordance with the second embodiment of theinvention.

FIG. 4A is a schematic of a first embodiment of a drive voltage controlcircuit in accordance with the invention.

FIG. 4B is a schematic of a second embodiment of a drive voltage controlcircuit in accordance with the invention.

FIG. 5 is a schematic of a first drive current control circuit inaccordance with the invention.

FIG. 6 is a schematic of a second drive current control circuit inaccordance with the invention.

FIG. 7 is a schematic of a third drive current control circuit inaccordance with the invention.

FIG. 8 is a schematic of a microbeamformer portion of a prior artultrasonic transducer.

Described below are several techniques for apodization control of amicrobeamformer of an ultrasonic transducer. These techniques share acommon goal of improving the ability to shape the transmit beams byadjusting the drive voltage or drive current provided to each transduceror to a plurality of transducers in a microbeamformer patch. Thetechniques can be used independently or to the extent possible, incombination with one another.

In a first embodiment of a system for apodization control of amicrobeamformer shown FIG. 1, the voltage provided to eachmicrobeamformer patch is adjustable. A mainframe beamformer 10 of theultrasonic imaging system includes a plurality of mainframe channels 12,each mainframe channel 12 including a mainframe transmitter 14 and amainframe receiver 16. A coaxial cable 18 connects each mainframechannel 12 to a respective sub-array beamformer also referred to hereinas a microbeamformer patch 20. Each patch 20 includes a plurality ofmicrobeamformer channels 22 each of which includes a microbeamformertransmitter 24, a transmitter timing control circuit 24A, a transducer26 which is driven by a signal from the microbeamformer transmitter 24,a microbeamformer receiver 28 which receives signals from the transducer26 and a delay element 30A. Instead of a coaxial cable 18, other cablesand electrical connection components as known in the art can be used. Adelay element 30B is often usually provided for the microbeamformertransmitters 24.

Each mainframe transmitter 14 provides a power supply voltage to therespective patch 20 via the cable 18. Thus, both the mainframetransmitter 14 and the mainframe receiver 16 are connected via a node 32to the cable 18. By coupling each of the mainframe transmitters 14 to arespective patch 20, an individually adjustable voltage can be providedto the microbeamformer transmitters 24 in the microbeamformer channels22 of each patch 20 to therefore enable control of the microbeamformerchannels 22 in each patch 20 and provide any desired shaping of thetransmit beam generated by the transducers 26 in each patch 20.

In operation, a unipolar pulse is transmitted by the mainframetransmitters 14 to the microbeamformer patches 20, more specifically tothe microbeamformer transmitters 24 in each microbeamformer patch 20,for at least the duration of each transmit burst. The voltage of theunipolar pulse is intermittently applied by the microbeamformertransmitters 24 in each microbeamformer patch 20 to create unipolarpulses to the transducers 26 in each microbeamformer patch 20. The timeat which the microbeamformer transmitters 24 drive the transducers 26high or low, and how many such pulses occur, is controlled uniquely andindividually within each of the microbeamformer channels 22, but theamplitude of the pulses is set by the voltage applied by the mainframetransmitters 14 which is the same for all of the microbeamformertransmitters 24 in one patch. Thus, both the mainframe transmitters 14and the microbeamformer transmitters 24 contribute to the drivingsignals for the transducers 26.

Diodes 34 are added in series with the transmitter power supply branch,both in the mainframe channels 12 and in the microbeamformer patches 20,to isolate the mainframe transmitters 14 and the microbeamformertransmitters 24 from the signal path during the receive mode. Althoughas shown in FIG. 1, one set of diodes 34 is arranged in eachmicrobeamformer patch 20 to isolate all of the microbeamformertransmitters 24 in that microbeamformer patch from the signal pathduring the receive mode, in the alternative, several sets of diodescould be used. A second set of diodes 34 is arranged in each mainframechannel 12 between the mainframe transmitter 14 and the node 32. Insteadof diodes 34, other isolation devices can be used as known to thoseskilled in the art.

Switches 36 are provided in the receiver branch, both in the mainframechannels 12 and in the patches 20, to add high-voltage protection to theinput of the mainframe receiver 16 and the output of the microbeamformerreceiver 28 so that they can withstand transmit events. Although asshown in FIG. 1, one switch 36 is arranged in each microbeamformerchannel 22 in each patch 20, alternative placements of switches could beused in the patches 20 so long as high-voltage protection is added tothe output of the microbeamformer receivers 28.

Another embodiment of a system for apodization control in accordancewith the invention has a simpler construction than the embodiment shownin FIG. 1 in that it does not require additional diodes and receiverinput/output protection, e.g., switches. Although the placement of thediodes and receiver output protection provides advantages over prior artapodization control techniques, it adds additional parts and controlcomplexity.

Referring now to FIG. 2, this embodiment does not provide for individualapodization for each patch 20 but rather provides for independentapodization for a plurality of discrete groups or clusters 38A,38B ofpatches 20 (only two of which are shown in FIG. 2). In this embodiment,the patches 20 are grouped into a plurality of clusters 38A,38B, with atleast one and possibly all of the clusters 38A,38B having a plurality ofpatches 20. Each cluster 38A,38B has a power supply voltage 40A,40Bwhich enables the transmit voltage of the patches 20 in that cluster 38Ato be independent of the transmit voltage of the patches 20 in the othercluster(s) 38B. In FIG. 2, only one microbeamformer channel 22 is shownin each patch 20 for simplicity, but each patch 20 is understood tocomprise multiple microbeamformer channels 22 (for example as shown inFIG. 1).

A coaxial cable 42 connects each power supply voltage 40A,40B to a node44 of the respective cluster 38A,38B which is hardwired to themicrobeamformer transmitters 24 of each patch 20 in that cluster.Instead of a coaxial cable 42, other cables or electrical connectioncomponents can be used.

This embodiment impacts the microbeamformer IC architecture in that itwould have to be designed to provide a plurality of separate powersupplies to the die (but only one to each patch), and to have theseparate power supplies arranged in a manner that provides usefulapodization. For example, FIG. 3 illustrates a potential arrangement forassigning patches 20 to clusters 38 in which 16 dies for formingintegrated circuits 46,48 are used (the double lines delineate theboundaries between the 16 ICs). The 128 patches 20 are grouped intoeight clusters 38 (by number). The four ICs 46 in the center two rowsuse all eight power supplies, while the other ICs 48 use fewer. However,the ICs 46,48 can still be identical if the supply voltage for eachpatch 20 is available as an input pin/pad in which case, the allocationof the supply voltage to each patch 20 can take place at the level ofinterconnect between the ICs 46,48 and the cabling where it is easier tocustomize. Thus, in this embodiment, a number of dedicated powersupplies are hardwired to the clusters 38 each of which comprises atleast one and preferably a plurality of patches 20.

The power supply voltages 40A,40B provided in the mainframe may be anyknown construction for providing regulatable power to themicrobeamformer transmitters 24.

Another embodiment of a system for apodization control in accordancewith the invention has a small number of high voltage rails (e.g., two)supplied to every microbeamformer transmitter. The transmitter can thenbe commanded to choose between these supplies when it drives theassociated transducer. As shown in FIG. 4A, two HV rails 88,90 areconnected to a single PMOS pull-up device or transistor 92A. HV rail 88(designated HV1) has a higher voltage than HV rail 90 (designated HV2).A high voltage switch 94 is interposed between HV rail 88 and thepull-up device 92A while a diode 96 is interposed between the HV rail 90and the pull-up device 92A. The pull-up device 92A is connected at anode 98 to the transducer 26. A pull-down device or transistor 92B isconnected to the node 98 to discharge the transducer 26.

By setting HV2 at a voltage below HV1, either voltage supply can beselected by activation of the single switch 94 on a per-element (orper-patch) basis to create unipolar pulses with amplitudes set bywhichever voltage supply is selected. In one possible operationalembodiment, transducers 26 near the periphery of the transmit aperturewill have the lower HV2 supply selected. The diode 96 could also be aswitch similar to switch 94, but would require additional circuits toturn on the device. More than two HV supplies can be supported byincluding additional switches and associated supplies in parallel withswitch 94.

Instead of connecting two HV rails to a single pull-up device, one via aswitch and the other via a diode as shown in FIG. 4A, it is possible touse a separate pull-up device 92A for each HV rail 88,90 as shown inFIG. 4B. The pull-up devices 92A act as switches in this embodiment soseparate switches (element 94 in FIG. 4A) are not required.

The technique of providing multiple voltage sources and switchingbetween the voltage sources using a pull-up device as disclosed in FIG.4A may be applied in all situations when voltage is provided to themicrobeamformer transmitters 24. Thus, this technique may be used withthe power supply clustering techniques discussed above with reference toFIGS. 1-3. For example, two or more HV rail supplies can be provided foreach patch to enable the voltage to that patch to be switched betweenthe HV supplies. In this manner, any of the supply voltages can be usedfor a particular transmit aperture on a patch by patch basis. Inaddition, the multiple HV rail supplies can be provided to eachmicrobeamformer transmitter 24 within a single patch 20 so that any ofthe supply voltages can be used for a particular transmit aperture on atransducer 26 by transducer 26 basis.

In some embodiments of the invention, in addition to or instead ofproviding for regulation of the voltage to the microbeamformertransmitters 24 in order to regulate the drive signals to thetransducers 26, it is possible to regulate the drive current provided bythe microbeamformer transmitters 24. In conventional microbeamformers,only the voltage provided by the microbeamformer transmitters 24 isregulated. However, by using a drive current model, the sensitivity ofthe transducers 26 to current is exploited. That is, rather than drivingthe transducers 26 solely with voltage excitation (with unlimitedcurrent), the transducers 26 are driven with a current, to the extentthat the voltages involved can be tolerated by the transducers 26. Inthis manner, very small driver circuits can be formed and integratedinto an IC arranged in close proximity to the transducers 26. In suchhighly integrated microbeamformers, there is no need for a large cableintervening between the drive circuits and the transducers 26 to becharged and discharged, thereby further improving efficiency of themicrobeamformers in accordance with the invention.

Drive current circuits used in the invention generally compriseswitchable current sources and sinks which enable a variation in thecurrent provided to the transducers 26. Since current into and out ofthe transducers 26 roughly corresponds to acoustic velocity, differentacoustic signals may be synthesized by varying the drive current.

A first embodiment of a current-controlled ultrasonic microbeamformertransmitter 24 is shown in FIG. 5 and includes a digital to analogconverter (DAC) cell 50 having a plurality of switchable current sourceseach including a pull-up device 52 connected at one end to the inputvoltage and a respective switch 54 connected to the other end of thepull-up device 52. Switches 54 are connected via an output node 56 tothe transducer 26. The transducer 26 is largely capacitive and is thusrepresented by a capacitor 58. The switches 54 are controlled in abinary fashion to provide up to eight different current outputs thatwhen the current outputs pass through the pull-up devices 52, eightdifferent currents are provided at the output node 56.

In view of the capacitance of the transducer 26, the voltage developedacross the load continues to grow if any of the pull-up devices 52 areenabled. To discharge the capacitor 58, a plurality of switchablecurrent sinks are provided, each current sink including a pull-downdevice 60 and a respective switch 62. The current sinks are controlledto provide up to eight different discharge currents. For optimalefficiency, it is useful not to allow pull-up devices 52 and pull-downdevices 60 to both be enabled at the same time. The voltage at theoutput node 56 is a function of the cumulative charge transfers on andoff the plate of the capacitor 58 and is thus a function of the currents52 and 60 and the durations that these devices are enabled to drive node56.

The pull-up devices 52 and pull-down devices 60 may be high voltageMOSFET devices that operate in saturation. The current passing throughan “ON” MOSFET is proportional to its width. As such, to implement theDAC cell 50, three high voltage PMOSFET pull-up devices 52 are needed,preferably with different widths, for example with widths 1×, 2×, and4×, and three NMOSFET pull-down devices 60 with corresponding widths(1×, 2×, and 4×). Alternatively, multiple pull-up or pull-down devicesin parallel with common gate connections can be used to provide thesedifferent drive currents. The manner in which the gates of the MOSFETdevices 52, 60 are controlled and the manner in which similar pull-upand pull-down currents can be obtained would be readily ascertainable toone skilled in the art. It should be also readily apparent to oneskilled in the art that the series switches 54, 62 could be eliminatedif the gates of the current source/sink MOSFET devices 52, 60 themselveswere driven to disable the current flow to/from node 56. Other pull-upand pull-down devices (e.g., bipolar transistors) known to those skilledin the art could also be used in the DAC cell 50.

The foregoing construction of DAC cell 50 applying drive current controlcircuitry is especially advantageous for controlling ultrasoundtransducers and in particular microbeamformer transducers since the DACcells 50 occupy less size than comparable voltage control circuitry(which requires large source/sink currents) and have better suited powerlimitations. Another advantage is that since there is no tuning networkor cable capacitance between the driver and the microbeamformertransducers (as there is in normal cabled transducers), the acousticresponse is much more predictable.

A further modification of drive current control circuitry is based onthe recognition that the total power provided to the transducers 26, andthus transmitted into the body, is a function not only of drive currentbut also of time. Accordingly, it is possible to use a high-voltage DACcell such as the DAC cell 50 described above with reference to FIG. 5,with pulse-width modulating controls to generate different transmitwaveforms. The pulse-width modulation concept can also be used with asingle current source/sink pair where the times that these currents areasserted can control the output acoustic amplitude.

FIG. 6 shows a DAC cell 70 applying pulse width modulation. DAC cell 70includes a single pull-up device 72 and a single pull-down device 74,each provided with a constant current and which are enabled fordifferent amounts of time. This provides the transducer connected to theoutput node 76 with different pulses of current that can synthesizedifferent waveform amplitudes or shapes.

A control computer or processor 78 is provided to specify events for thecontrol changes and the events for each line and frame of the imagingprocess, and may also optionally generate such events. The controlcomputer 78 directs the required event commands to a timing generator80, and optionally to the mainframe transmitters. The mainframetransmitters can provide a full power waveform to the DAC cell 70 viapower supply rails as well as in any of the constructions describedabove. The ultrasound imaging system can also include other componentsfor generating the event commands for providing a desired transmitwaveform as known in the art.

The timing generator 80 determines the number of pulses in each burst,the pulse train frequency, the pulse width and the delay (used forfocusing) and generates an appropriate timing signal which will causethe DAC cell 70 to generate the desired waveform from the power supplyupon receiving the timing signal.

More specifically, the timing generator 80 generates a timing signal orpulse width modulation signal for the switches 82, 84. The extent towhich switch 82 is on or off, i.e., the time in which the switch 82 ison, determines the width of the current pulse from the pull-up device72. The extent to which switch 84 is on or off, i.e., the time in whichthe switch 84 is on, determines the width of the current pulse to groundthrough the pull-down device 74.

The DAC cell 70 is shown with a single pull-up device 72 and a singlepull-down device 74. As shown in FIG. 7, it is also possible toconstruct a DAC cell 86 with multiple pull-up and pull-down deviceswhich provide a plurality of different currents. Thus, if MOSFET devicesare used as the pull-up devices 72 and the pull-down devices 74, thewidths of the MOSFET devices are different, for example, x, 2× as shown.In this manner, different currents are provided by the differentcombinations of opening and closing the switches 82,84.

Control of the timing of the pull-up and pull-down devices 72, 74 whenthese devices 72, 74 drive the transducer is complex but can beascertained by one skilled in the art. The capacitive load should beadequately controlled from one channel to the next so that consistentacoustic outputs can be obtained.

The current-mode scaling/apodization techniques discussed above withreference to FIGS. 4-6 are adequate when the voltage developed at theoutput of the microbeamformer transmitter 24 does not reach the highvoltage supply rail (the HV rail). When the output voltage approachesthat of the HV rail, the PMOSFETS pull-up devices 72 come out ofsaturation and eventually stop conducting current when the HV railequals the voltage on the transducer 26. Thus, it is a drawback that thecurrent modulation and current-pulse-width modulation techniques areineffective when the transducer voltage charges all the way to the HVrail. However, to maximize acoustic power output, a charge as close aspossible to the HV rail is desired. Since there is inherently someuncertainty in both the driver current and actual element load, it isvery difficult to obtain maximum output acoustic power and still havegood control over the transmit waveforms.

The current-mode techniques discussed above with reference to FIGS. 5-7are sensitive to element load and to driver variability resulting inoperational drawbacks and thus are not suitable for all applications. Toovercome these drawbacks, the power (voltage) supply control techniquespresented earlier in FIGS. 1-4 can also be used in conjunction with thecurrent-mode techniques discussed above. For example, the pull-up device92A and pull-down device 92B in the dual HV rail circuit in FIG. 4Acould implement current sources 72 and 74 in FIG. 7. The gates of thepull-up device 92A and pull-down device 92B, labeled with UP and DOWN inFIG. 4, could be driven with appropriate pulse-width modulated signalsso as to supply fixed source and sink currents to the transducer 26 forspecific durations of time. If the pulse duration is not long enough forthe output node 98 to be driven to the selected HV rail (either 88 or90), then the pulse-width modulation techniques may be used. On theother hand, if the pulse duration is long enough to fully charge theoutput node 98 to the selected HV rail, then the voltage supplytechniques (clustering etc.) may be used.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to these preciseembodiments, and that various other changes and modifications may beeffected therein by one of ordinary skill in the art without departingfrom the scope or spirit of the invention.

1. A system for apodization control of a microbeamformer, comprising: amainframe beamformer having a plurality of mainframe channels, each ofsaid mainframe channels including a mainframe receiver and a mainframetransmitter for transmitting a pulsed power supply voltage; a pluralityof microbeamformer patches, each of said microbeamformer patchesincluding a plurality of microbeamformer channels, each of saidmicrobeamformer channels including a transducer, a microbeamformertransmitter and control circuit for driving said transducer, amicrobeamformer receiver for receiving signals from said transducer anda delay element for delaying the received transducer signals, saidmicrobeamformer transmitters in each of said microbeamformer patchesbeing connected to a common node; a plurality of separate electricalconnectors, each of said connectors connecting said common node in arespective one of said microbeamformer patches to a respective one ofsaid mainframe channels such that said mainframe transmitter in saidrespective mainframe channel provides the pulsed power supply voltage toall of said microbeamformer transmitters in said microbeamformer patchvia said connector; isolating means for isolating said microbeamformertransmitters and said mainframe transmitters when said microbeamformerreceivers and said mainframe receivers are operative; and protectionmeans for protecting said microbeamformer receivers and said mainframereceivers when said microbeamformer transmitters and said mainframetransmitters are operative.
 2. The system of claim 1, wherein saidisolation means are arranged between each of said connectors and saidmainframe transmitter in the respective one of said mainframe channelsand between each of said connectors and said common node of therespective one of said microbeamformer patches.
 3. The system of claim1, wherein said isolation means comprise diodes.
 4. The system of claim1, wherein said protection means are arranged between each of saidconnectors and said mainframe receiver in the respective one of saidmainframe channels and between each of said connectors and saidmicrobeamformer receivers in each of said microbeamformer channels. 5.The system of claim 1, wherein said protection means comprise switches.6. The system of claim 1, wherein said connectors comprise cables. 7.The system of claim 1, wherein said mainframe transmitter in each ofsaid mainframe channels is arranged to transmit a unipolar pulse for theduration of each transmit burst by said microbeamformer transmitters inthe respective one of said microbeamformer patches
 8. The system ofclaim 7, wherein said mainframe transmitters provide the power supplyvoltage for each of said microbeamformer transmitters in the respectiveone of said microbeamformer patches and thus defines the pulse amplitudeapplied to each of said transducers in the respective one of saidmicrobeamformer patches.
 9. The system of claim 8, wherein saidmainframe transmitters are arranged to vary the transmitted voltage. 10.The system of claim 1, wherein said microbeamformer transmitter in atleast one of said microbeamformer channels comprises: a plurality ofswitchable current sources, each of said current sources comprising apull-up device arranged to receive the pulsed power supply voltage and aswitch interposed between said pull-up device and said transducer ofsaid microbeamformer channel; and a plurality of switchable currentsinks, each of said current sinks comprising a pull-down device and aswitch interposed between said pull-down device and ground, saidpull-down devices being arranged to discharge said transducer throughsaid switches.
 11. The system of claim 10, wherein said pull-up devicesare PMOSFET devices operating in saturation, said PMOSFET devices havingdifferent widths and providing a current proportional to the width. 12.The system of claim 10, wherein said pull-down devices are NMOSFETdevices having different widths and providing a current proportional tothe width.
 13. The system of claim 1, wherein said microbeamformertransmitter in at least one of said microbeamformer channels comprises:at least one switchable current source, each of said at least onecurrent source comprising a pull-up device arranged to receive thepulsed power supply voltage and a switch interposed between said pull-updevice and said transducer of said microbeamformer channel; at least oneswitchable current sink, each of said at least one current sinkcomprising a pull-down device and a switch interposed between saidpull-down device and ground, said pull-down device being arranged todischarge said transducer through said switch; and control means forcontrolling said switches in each of said at least one current sourceand said at least one current sink to pulse width modulate a transmitwaveform generated by said microbeamformer transmitter.
 14. The systemof claim 13, wherein said control means comprise a timing generator forgenerating signals to control said switch in each of said at least onecurrent source and said at least one current sink and a control computerfor controlling said timing generator.
 15. The system of claim 13,wherein said at least one current source comprises a plurality ofcurrent sources, said pull-up devices in said current sources beingPMOSFET devices operating in saturation and having different widths andproviding a current proportional to the width.
 16. The system of claim15, wherein said at least one current sink comprises a plurality ofcurrent sinks, said pull-down devices in said current sinks beingNMOSFET devices operating in saturation and having different widths andproviding a current proportional to the width.
 17. The system of claim16, wherein said control means comprise a timing generator forgenerating signals to control said switches in each of said currentsources and said current sinks and a control computer for controllingsaid timing generator.
 18. A system for apodization control of amicrobeamformer, comprising: a plurality of microbeamformer patches,each of said microbeamformer patches including a plurality ofmicrobeamformer channels, each of said microbeamformer channelsincluding a transducer, a microbeamformer transmitter and controlcircuit for driving said transducer, a microbeamformer receiver forreceiving signals from said transducer and a delay element for delayingthe received transducer signals, said microbeamformer patches beinggrouped into a plurality of clusters such that each of said clustersincludes a plurality of said microbeamformer patches, saidmicrobeamformer transmitters in each of said clusters being connected toa common node; a mainframe beamformer having a plurality of individuallyadjustable power supplies and a plurality of mainframe channels, each ofsaid channels including a mainframe receiver; first connecting means forconnecting said microbeamformer receivers in each of saidmicrobeamformer patches to a respective one of said mainframe channels;and second connecting means for connecting each of said clusters to arespective one of said power supplies.
 19. The system of claim 18,wherein said first and second connecting means comprise cables.
 20. Thesystem of claim 18, wherein the power supply voltage applied to each ofsaid clusters is independently adjustable.
 21. The system of claim 20,wherein the power supply voltage applied to each of said clusters isadjustable to a different voltage for each pulse repetition interval.22. The system of claim 18, wherein said microbeamformer transmitter inat least one of said microbeamformer channels comprises: a plurality ofswitchable current sources, each of said current sources comprising apull-up device arranged to receive the pulsed power supply voltage and aswitch interposed between said pull-up device and said transducer ofsaid microbeamformer channel; and a plurality of switchable currentsinks, each of said current sinks comprising a pull-down device and aswitch interposed between said pull-down device and ground, saidpull-down devices being arranged to discharge said transducer throughsaid switches.
 23. The system of claim 22, wherein said pull-up devicesare PMOSFET devices operating in saturation, said PMOSFET devices havingdifferent widths and providing a current proportional to the width. 24.The system of claim 22, wherein said pull-down devices are NMOSFETdevices having different widths and providing a current proportional tothe width.
 25. A microbeamformer channel, comprising: a transducer; amicrobeamformer transmitter provided with a power supply voltage fordriving said transducer; a control circuit to drive the microbeamformertransmitter; a microbeamformer receiver for receiving signals from saidtransducer; and a delay element for delaying the received transducersignals, said microbeamformer transmitter comprising a plurality ofswitchable current sources, each of said current sources comprising apull-up device arranged to receive the pulsed power supply voltage and aswitch interposed between said pull-up device and said transducer ofsaid microbeamformer channel; and a plurality of switchable currentsinks, each of said current sinks comprising a pull-down device and aswitch interposed between said pull-down device and ground, saidpull-down devices being arranged to discharge said transducer throughsaid switches.
 26. The system of claim 25, wherein said pull-up devicesare PMOSFET devices operating in saturation, said PMOSFET devices havingdifferent widths and providing a current proportional to the width. 27.The system of claim 25, wherein said pull-down devices are NMOSFETdevices having different widths and providing a current proportional tothe width.
 28. A microbeamformer channel, comprising: a transducer; amicrobeamformer transmitter provided with a power supply voltage fordriving said transducer; a control circuit to drive said microbeamformertransmitter; a microbeamformer receiver for receiving signals from saidtransducer; and a delay element for delaying the received transducersignals, said microbeamformer transmitter comprising at least oneswitchable current source, each of said at least one current sourcecomprising a pull-up device arranged to receive the pulsed power supplyvoltage and a switch interposed between said pull-up device and saidtransducer of said microbeamformer channel; at least one switchablecurrent sink, each of said at least one current sink comprising apull-down device and a switch interposed between said pull-down deviceand ground, said pull-down device being arranged to discharge saidtransducer through said switch; and control means for controlling saidswitch in each of said at least one current source and said at least onecurrent sink to pulse width modulate a transmit waveform generated bysaid microbeamformer transmitter.
 29. The microbeamformer channel ofclaim 28, wherein said control means comprise a timing generator forgenerating signals to control said switch in each of said at least onecurrent source and said at least one current sink and a control computerfor controlling said timing generator.
 30. The microbeamformer channelof claim 28, wherein said at least one current source comprises aplurality of current sources, said pull-up devices in said plurality ofcurrent sources being PMOSFET devices operating in saturation, saidPMOSFET devices having different widths and providing a currentproportional to the width.
 31. The microbeamformer channel of claim 30,wherein said least one current sink comprises a plurality of currentsinks, said pull-down devices in said plurality of current sinks beingNMOSFET devices operating in saturation, said NMOSFET devices havingdifferent widths and providing a current proportional to the width. 32.The microbeamformer channel of claim 31, wherein said control meanscomprise a timing generator for generating signals to control saidswitches in each of said current sources and said current sinks and acontrol computer for controlling said timing generator.
 33. A system forapodization control of a microbeamformer, comprising: a mainframebeamformer having a plurality of mainframe channels, each of saidmainframe channels including a mainframe receiver; a plurality ofmicrobeamformer channels, each of said microbeamformer channelsincluding a transducer, a microbeamformer transmitter for driving saidtransducer, a microbeamformer receiver for receiving signals from saidtransducer and a delay element for delaying the received transducersignals; supply means for selectively supplying one of a plurality ofdifferent voltages to said microbeamformer transmitters.
 34. The systemof claim 33, wherein said supply means comprise: a plurality of highvoltage rails, a first one of said rails providing a higher voltage thana second one of said rails; a pull-up device interposed between saidrails and said transducer; a switch interposed between said first railand said pull-up device, said switch being controlled such that when inan on position, the voltage provided to said pull-up device is that ofsaid first rail and when in an off position, the voltage provided tosaid pull-up device is that of said second rail; and a diode interposedbetween said second rail and said pull-up device.
 35. The system ofclaim 34, further comprising at least one current sink for dischargingsaid transducer, said at least one current sink comprising a pull-downdevice connectable to ground.
 36. The system of claim 34, wherein saidmicrobeamformer channels are grouped into a plurality of microbeamformerpatches, each of said microbeamformer patches including a plurality ofsaid microbeamformer channels, said microbeamformer transmitters in eachof said microbeamformer patches being connected to a respective commonnode with each of said rails such that the same voltage is provided toall of said microbeamformer transmitters in said microbeamformer patchby said supply means.
 37. The system of claim 34, wherein saidmicrobeamformer channels are grouped into a plurality of microbeamformerpatches, each of said microbeamformer patches including a plurality ofsaid microbeamformer channels, and said microbeamformer patches aregrouped into a plurality of clusters, each of said clusters including aplurality of said microbeamformer patches, said microbeamformertransmitters in each of said clusters being connected to a respectivecommon node with each of said rails such that the same voltage isprovided to all of said microbeamformer transmitters in said cluster bysaid supply means.
 38. The system of claim 34, wherein said rails areconnected to each of said microbeamformer transmitters, said pull-updevice, said switch and said diode being arranged in each of saidmicrobeamformer transmitters.
 39. The system of claim 33, wherein saidsupply means comprise: a plurality of high voltage rails providingdifferent voltages; and a plurality of pull-up devices, each of saidpull-up devices being interposed between a respective one of said railsand said transducer.